Airway Disease

Respiratory diseases in horses can be grouped into two general categories – infectious and noninfectious. Infectious respiratory diseases include bacterial and viral infections. The most common of the non-infectious respiratory conditions are Small Airway Inflammatory Disease (SAID) and its more severe form known as “heaves” or Recurrent Airway Obstruction (RAO).

SAID v.s. RAO

As its name implies SAID is an inflammatory condition of the very small airways deep within the horse’s lungs, similar to asthma in humans. RAO or “heaves” is a chronic, recurrent condition characterized by obstruction of the airways with mucus and constriction of the airways that result in more severe clinical signs, similar to emphysema in humans.

What causes SAID?

Dust and air pollutants in the horse’s environment enter the small airways during exercise. These substances cause irritation to the lining of the lungs which precipitates inflammation.

Inflammation in the lungs causes the accumulation of mucus and cellular debris within the airways which also leads to thickening of the walls of the airways and constriction of the tiny muscles of the airways.

The mucus traps foreign particles and is moved up the lungs and finally is coughed out in a “normal” horse in an “average environment”.

Certain horses mount excessive inflammatory responses to “normal” amounts of dusts and pollutants. This results in the typical clinical signs of SAID.

Keep in mind that horses have tremendous lung capacity. For example, a racehorse running at top speed breathes about 120 times per minute, moving about 12-15 liters of air per breath or 1,400-1,800 liters per minute through the lungs. A horse at rest uses only a mere fraction of his/her full lung capacity. Therefore, the degree of clinical signs you may observe will vary depending on the horse’s endeavors.

Signs of RAO/”heaves”

These horses can have acute attacks of severe respiratory difficulty (similar to an acute asthma attack in people) with or without exertion.

Diagnosis

SAID is diagnosed based on history, clinical signs, physical exam findings and diagnostic tests including:

Endoscopy during or after exercise which often reveals mucus in the trachea

Bronchoalveolar lavage – A long, flexible tube is passed via the nostrils down into the lungs. Sterile saline is flushed through the tube and immediately retrieved via suction. The fluid is then analyzed and the specific types of inflammatory cells can be identified.

RAO is most often diagnosed simply based on history, clinical signs and physical exam findings. It is often not necessary to perform the other tests described above due to the more severe nature of the disease, though testing can help identify the most appropriate treatment

Treatment

Medical treatments as well as environmental management strategies are employed. Medical treatment includes drugs to control airway inflammation and bronchoconstriction. Oral corticosteroids such as prednisolone (not to be confused with the steroids used by some athletes) are effective at reducing airway inflammation. Inhaled steroids are very effective and allow delivery of the drug directly to the lungs while decreasing the potential adverse side effects (immunosuppression, laminitis, adrenal gland suppression). Bronchodilators provide relief of airway constriction making it easier to breathe and decreasing coughing.

Oral anti-histamines do not work well in horses with these conditions.

Once the clinical signs have been controlled medically, environmental management becomes the MOST IMPORTANT part of long term treatment. Our recommendations include:

Decrease exposure to dust and other pollutants (including ammonia from urine soaked bedding).

The best place for a “heavey” horse to live is outside with a run-in shed for shelter. If this is not possible, an end stall, near an open door will help with ventilation.

Even in the most beautiful barns, hay storage overhead can cause major issues for a horse with one of these conditions. In fact, even the most beautiful hay can cause a problem. These horses have “hyper-reactive” lungs. Even if you take a big sniff of the hay and pronounce it dust free, it can still carry enough dust and allergens to cause an issue. Soaking hay thoroughly and feeding it off the ground will reduce the amount of hay dust inhaled during eating. Your horse may also benefit from wetting his grain pellets or switching to bagged hay such as Dengie, Totally Timothy, or other fiber sources such as soaked beet pulp or hay cubes. In many cases, it is necessary to remove all hay from a severely affected horse’s diet.

Bedding can also vary tremendously in the amount of dust and mold spores it contains.

Prognosis

SAID can be treated effectively in most cases. As stated above however, certain horses are more prone to having hyper-reactive airways which predispose them to repeated episodes. This is why environmental management is so important. In many patients there is a seasonal component with episodes being worse in spring and fall due to environmental allergens. Some horses are worse in the winter when they spend more time inside the barn. SAID that is not effectively managed can develop into RAO. RAO that is not effectively controlled can ultimately result in irreversible scarring and fibrosis of the lungs.